Molecular basis for the maintenance of lipid asymmetry in the outer membrane of Escherichia coli

Abstract

A distinctive feature of the Gram‐negative bacterial cell envelope is the presence of an asymmetric outer membrane (OM), where lipopolysaccharides (LPS) and phospholipids (PLs) reside in the outer and inner leaflets, respectively. This unique lipid asymmetry renders the OM impermeable to external insults, thus allowing survival of bacteria in harsh environments. In Escherichia coli, the OmpC‐Mla system is responsible for maintenance of OM lipid asymmetry. Osmoporin OmpC and the OM lipoprotein MlaA form a complex proposed to remove PLs from the outer leaflet of the OM. How this complex is organized in the OM to perform this function is not known. In this report, we define the molecular architecture of the OmpC‐MlaA complex to gain insights into its function in PL transport. We show that MlaA sits entirely within the OM lipid bilayer, and interacts with the OmpC trimer at its dimeric interfaces. Molecular dynamics simulations reveal a membrane‐spanning hydrophilic channel within MlaA, suggesting a path for PL translocation across the OM. We demonstrate that a hydrophobic hairpin loop adjacent to this putative channel is critical for modulating the activity of MlaA; restricting flexibility of this structure significantly perturbs the function of the complex. Finally, we establish that OmpC plays an active role in maintaining OM lipid asymmetry together with MlaA. Our work provides a glimpse into the molecular mechanism of how the OmpC‐MlaA complex may extract PLs from the outer leaflet of the OM, and highlights key features that could be exploited in the development of future antimicrobial drugs.Support or Funding InformationNational University of Singapore Graduate School for Integrative Sciences and Engineering scholarship (to J.Y.).Singapore Ministry of Education Academic Research Fund Tier 3 grant (MOE2012‐T3‐1‐008) grant to (P.J.B.).Singapore Ministry of Education Academic Research Fund Tier 1 and Tier 2 (MOE2013‐T2‐1‐148) grants (to S.‐S.C.).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.